1. Field of the Invention
The present invention relates to a method for manufacturing a nickel alloy electroplated cold-rolled steel sheet excellent in press-formability and phosphating-treatability.
2. Prior Art Statement
In general, a cold-rolled steel sheet for automobiles or electric appliances is formed into a prescribed shape by means of a large-capacity press. With a view to achieving a larger automobile body, reducing air resistance during running of a car, and achieving an exterior view of a better style, it is the present practice to form fenders, doors and rear quarter portions into rounded shapes.
From the point of view of economic merits and environmental protection, on the other hand, efforts are being made to reduce the weight of an automobile body so as to reduce the fuel consumption. In order to reduce the weight of the automobile body, it is necessary to decrease the thickness of a steel sheet which forms the automobile body, and this is also the case with a steel sheet such as an exposed panel that should be subjected to a deep drawing. The steel sheet for an exposed panel requires a satisfactory dent resistance and shape freezability. It is therefore necessary to use a high-strength steel having a thin thickness for the exposed panel. In order to form a thin and high-strength cold-rolled steel sheet by the deep drawing, it is necessary to previously increase the wrinkle inhibiting force of the steel sheet by means of a powerful press so as to prevent wrinkles from producing on the cold-rolled steel sheet during the press forming.
Annealing applied to the cold-rolled steel sheet for the purpose of recrystallization of crystal grains subjected to a serious strain during the cold rolling thereof, is applicable either by a continuous annealing or a box annealing.
An ordinary low-carbon aluminum-killed steel has been used as a material for a mild cold-rolled steel sheet for deep drawing. A low-carbon aluminum-killed steel containing silicon, manganese and phosphorus has been used as a material for a high-strength steel sheet for deep drawing. The box annealing has been applied for the purpose of annealing the above-mentioned mild cold-rolled steel sheet for deep drawing and high-strength steel sheet for deep drawing. The box annealing is characterized by a long heating time, a long cooling time, easy growth of crystal grains, and the availability of a cold-rolled steel sheet having a high Lankford value;
A box-annealed steel sheet is exposed to a high temperature for a longer period of time than a continuous-annealed steel sheet. As a result, silicon, manganese and phosphorus contained in the box-annealed steel sheet are concentrated onto the surface of the steel sheet in the form of oxides. These oxides concentrated onto the surface of the steel sheet serve as a lubricant film during the press forming. In addition, the box-annealed steel sheet has a high Lankford value than that of the continuous-annealed steel sheet. Therefore, troubles such as press cracks hardly occur in the box-annealed steel sheet.
When the box-annealed steel sheet is press-formed and then subjected to a phosphating treatment, the elements contained in the steel sheet and the elements such as manganese concentrated onto the surface of steel sheet activate a phosphate film forming reaction, so that a dense and thin phosphate film is formed on the surface of the steel sheet. The phosphate film has a function of improving paint adhesivity and corrosion resistance after painting of the steel sheet.
Recently, however, it is becoming an increasingly usual practice to anneal a steel sheet by the continuous annealing for such reasons as the reduction of manufacturing processes, the improvement of production yield and labor saving. The known cold-rolled steel sheets suitable for the application of the continuous annealing treatment comprise an extra-low-carbon steel or a steel known as the interstiticial atoms free steel (hereinafter referred to as "IF steel").
In order to improve a Lankford value serving as an indicator of press-formability of an extra-low-carbon steel sheet, the following measure is taken: degassing the steel during the steelmaking step to reduce the carbon content to up to 100 ppm, and minimizing the contents of other impurity elements, thereby permitting rapid growth of crystal grains of steel.
The IF steel is produced by adding at least one of titanium and niobium to an extra-low-carbon steel, and fixing carbon and nitrogen acting as solid-solution elements by means of these added elements, thereby making it possible to obtain a higher Lankford value with a short-time continuous annealing.
Since the development of the above-mentioned extra-low carbon steel and IF steel, it is now possible to manufacture a cold-rolled steel sheet having a high Lankford value even by applying the continuous annealing.
However, the Lankford value of a cold-rolled steel sheet for deep drawing subjected to the continuous annealing (hereinafter referred to as the "continuous-annealed cold-rolled steel sheet") is equal or even superior to the Lankford value of a cold-rolled steel sheet for deep drawing subjected to the conventional box annealing (hereinafter referred to as the "box-annealed cold-rolled steel sheet"). However, the continuous-annealed cold-rolled steel sheet is easily susceptible to cracks during the press forming, and when worked into a complicated shape, more susceptible to the galling than the box-annealed cold-rolled steel sheet. As a result of various studies on causes thereof, it was revealed that, as shown in Table 1, there was a substantial difference in the value of frictional coefficient of the steel sheet surface between the continuous-annealed cold-rolled steel sheet and the box-annealed cold-rolled steel sheet. Table 1 shows values of frictional coefficient (.mu.) of the surface, Lankford values (r-value) and limiting drawing ratios (LDR) for the conventional continuous-annealed and box-annealed cold-rolled steel sheets, and Table 2 shows chemical compositions of the continuous-annealed and box-annealed cold-rolled steel sheets used in these studies.
TABLE 1 __________________________________________________________________________ Box-annealed cold-rolled steel sheet Continuous-annealed cold-rolled steel sheet (conventional; without plating) (conventional; without plating) Reduc- Heating Reduc- Heating tion temper- Frictional tion temper- Frictional Steel ratio ature r - coefficient Steel ratio ature r - coefficient grade (%) (.degree.C.) value (.mu.) LDR Remarks grade (%) (.degree.C.) value (.mu.) LDR Remarks __________________________________________________________________________ A 60 600 1.55 0.13 2.04 B 75 750 1.45 0.16 2.01 A 65 600 1.60 0.12 2.03 B 80 750 1.50 0.17 2.01 A 70 600 1.65 0.12 2.06 C 80 830 1.75 0.16 2.05 A 75 650 1.75 0.11 2.06 C 85 830 1.75 0.17 2.04 A 80 650 1.80 0.13 2.08 D 75 830 1.95 0.19 2.07 A 85 650 1.80 0.14 2.07 D 80 830 2.05 0.17 2.08 A 75 750 1.95 0.12 2.11 OCA D 80 830 2.00 0.18 2.06 decar- burized A 80 750 2.05 0.12 2.13 OCA D 85 830 2.00 0.19 2.08 decar- burized A 85 750 2.05 0.11 2.11 OCA E 75 830 2.05 0.17 2.07 decar- burized F 75 700 1.65 0.12 2.05 40 Kg E 80 830 2.20 0.18 2.10 High- strength steel E 80 830 2.25 0.18 2.10 E 85 830 2.25 0.19 2.08 G* 80 830 1.10 0.17 1.94 45 Kg High- strength steel __________________________________________________________________________ (OCA decarburized: open coil annealing decarburized) (*Only steel G hotrolled and coiled at a low temperature)
TABLE 2 __________________________________________________________________________ (wt. %) Steel grade C Si Mn P S Sol. A1 N Nb Ti Remarks __________________________________________________________________________ A 0.050 0.020 0.250 0.015 0.010 0.050 0.0030 -- -- Low carbon Al--K CC steel B 0.025 0.015 0.200 0.014 0.009 0.045 0.0031 -- -- Medium carbon Al--K CC steel C 0.003 0.012 0.150 0.014 0.010 0.038 0.0020 -- -- Extra-low-carbon Al--K CC steel D 0.003 0.012 0.130 0.015 0.008 0.037 0.0020 0.010 0.040 Extra-low-carbon Nb--Ti lF steel E 0.003 0.012 0.140 0.014 0.009 0.040 0.0020 -- 0.070 Extra-low-carbon Ti lF steel F 0.080 0.050 0.500 0.011 0.008 0.047 0.0035 -- -- Box-annealed 40 Kg high-strength steel G 0.032 0.350 2.200 0.040 0.005 0.030 0.0030 0.010 0.080 Continuous-annealed 45 Kg high-strength steel __________________________________________________________________________ (Al--K: aluminum killed; CC steel: continuously cast steel)
FIG. 1 is a graph illustrating the relationship between a Lankford value and a limiting drawing ratio, for a continuous-annealed cold-rolled steel sheet and a box-annealed cold-rolled steel sheet. In FIG. 1, the mark ".largecircle." represents the box-annealed cold-rolled steel sheet, and the mark ".DELTA." represents the continuous-annealed cold-rolled steel sheet. As shown in FIG. 1, the differences in the Lankford value and the limiting drawing ratio between the continuous-annealed and the box-annealed cold-rolled steel sheets are considered to be caused by the fact that a high frictional coefficient of the steel sheet surface as in the continuous-annealed cold-rolled steel sheet reduces lubricity between the steel sheet surface and the wrinkle inhibiting jig or the die, thus impairing a smooth flow of the material in the press die.
Now, the phosphating-treatability of the continuous-annealed cold-rolled steel sheet is described. Application of a phosphating treatment to the press-formed continuous-annealed cold-rolled steel sheet forms a phosphate film on the surface of the continuous-annealed cold-rolled steel sheet. Because the continuous-annealed cold-rolled steel sheet has only low contents of impurity elements, and the time of exposure of the steel sheet surface to a high temperatures during the annealing is far shorter than that in the box-annealed cold-rolled steel sheet, there is almost no concentration of the elements contained in the steel sheet onto the surface thereof. Consequently, there are only a very few cathodes to form precipitation nuclei of phosphate crystal grains on the surface of the continuous-annealed cold-rolled steel sheet, so that a phosphate film formed on the steel sheet surface comprises rough and coarse crystal grains.
FIG. 5 is an SEM (scanning electron microscope) micrograph showing the metallurgical structure of crystals of the phosphate film formed on the surface of the box-annealed cold-rolled steel sheet, and FIG. 6 is an SEM micrograph showing the metallurgical structure of crystals of the phosphate film formed on the surface of the continuous-annealed cold-rolled steel sheet. As shown in FIG. 6, the phosphate film formed on the surface of the continuous-annealed cold-rolled steel sheet has coarse and larger crystal grains than those formed on the surface of the box-annealed cold-rolled steel sheet shown in FIG. 5. The continuous-annealed cold-rolled steel sheet is therefore inferior in phosphating-treatability, paint adhesivity and corrosion resistance after painting to the box-annealed cold-rolled steel sheet.
The above-mentioned inferiority of the continuous-annealed cold-rolled steel sheet in phosphating-treatability is observed when pickling the steel sheet surface with an inorganic acid not only in the case of an extra-low-carbon steel but also in the case of an ordinary low-carbon aluminum-killed steel and a capped steel.
As a means to solve the problem regarding the inferior phosphating-treatability of the pickled continuous-annealed cold-rolled steel sheet, technologies of forming an alloy plating layer comprising phosphorus and at least one of nickel and niobium on the surface of the cold-rolled steel sheet have been proposed as follows:
An alloy plated extra-low-carbon steel sheet excellent in phosphating-treatability, as disclosed in Japanese Patent Provisional Publication No. 63-79,996 dated Apr. 9, 1988, which comprises:
an extra-low-carbon steel sheet containing carbon in an amount of up to 0.005 wt. %, at least one of titanium and niobium in an amount within a range of from 0.005 to 0.15 wt. % and the balance being iron and incidental impurities; and an alloy plating layer, formed on the surface of said extra-low-carbon steel sheet, comprising phosphorus and at least one of nickel and cobalt, the content of said phosphorus being within a range of from 1 to 30 wt. %, said alloy plating layer having a plating weight within a range of from 10 to 500 mg/m.sup.2 per surface of said extra-low-carbon steel sheet (hereinafter referred to as the "prior art 1").
According to the prior art 1, it is possible to obtain an alloy plated continuous-annealed cold-rolled steel sheet excellent in phosphating-treatability comprising an extra-low-carbon steel. This is attributable to the fact that phosphorus contained in the alloy plating layer promotes the cathodic reaction on the steel sheet surface, thus making it possible to obtain an excellent phosphating-treatability.
The prior art 1 has however the following problems.
in order for the continuous-annealed cold-rolled steel sheet to have a phosphating-treatability equal to that of the box-annealed cold-rolled steel sheet, it is necessary to adjust the number of initially precipitated nuclei of phosphate, i.e., the number of local cells produced on the steel sheet surface to a certain distribution density. For this purpose, it is important that the alloy particles comprising nickel and/or cobalt and phosphorus are precipitated into the alloy plating layer, and that the distribution density of the alloy particles is at least a certain value. According to the prior art 1, there is no description in this respect. An excellent phosphating-treatability cannot necessarily be obtained by only forming the alloy plating layer comprising nickel and/or cobalt and phosphorus on the steel sheet surface.
When the plating weight of the alloy plating layer comprising nickel and/or cobalt and phosphorus is over 100 mg/m.sup.2 per surface of the steel sheet, the coating ratio of the steel sheet surface by the alloy plating layer becomes higher, with a reduced distribution density of the precipitation nuclei of phosphate, and crystal grains of the phosphate film become coarser. As a result, the deposited amount of the phosphate film is insufficient relative to the prescribed value, leading to a poor paint adhesivity and a poor corrosion resistance after painting.
As it is difficult to plate phosphorus alone on the steel sheet surface, phosphorus is alloyed with nickel and/or cobalt for plating. Phosphorus has a function of increasing hardness of the alloy plating layer, facilitating the formation of an oil film on the sliding face of the steel sheet surface, and thus decreasing a frictional coefficient. However, a phosphorus content of over 15 wt. % seriously reduces the electrolytic efficiency upon electroplating, thus increasing the equipment cost for continuous annealing which requires a high-speed operation.
Because the increase in the plating weight of the alloy plating layer comprising nickel and/or cobalt and phosphorus leads to a lower phosphate-treatability of the cold-rolled steel sheet, it is necessary to minimize the plating weight of the above-mentioned alloy plating layer as far as possible. However, when the plating weight of the alloy plating layer is reduced, the frictional coefficient of the steel sheet surface increases, thus resulting in a poorer press-formability. An excellent press-formability cannot always be obtained therefore according to the prior art 1.
As a technology for improving phosphating-treatability and corrosion resistance of the cold-rolled steel sheet, the following cold-rolled steel sheet is proposed;
A nickel plated cold-rolled steel sheet excellent in phosphating-treatability and corrosion resistance, disclosed in Japanese Patent Provisional Publication No. 2-101,200 dated Apr. 12, 1990, which comprises:
A cold-rolled steel sheet; and a nickel plating layer, formed on the surface of said cold-rolled steel sheet, in which layer nickel particles are precipitated at a distribution density within a range of from 1.times.10.sup.12 to 5.times.10.sup.14 /m.sup.2, the plating weight of said nickel plating layer being within a range of from 1 to 50 mg/m.sup.2 per surface of said cold-rolled steel sheet, each of said nickel particles comprising metallic nickel and non-metallic nickel, having a thickness within a range of from 0.0009 to 0.03 .mu.m , adhering to the surface of said metallic nickel, and said nickel particles having a particle size within a range of from 0.001 to 0.3 .mu.m (hereinafter referred to as the "prior art 2").
According to the above-mentioned prior art 2, it is possible to form a dense and uniform phosphate film having a crystal grain size within a certain range, thereby making it possible to obtain a cold-rolled steel sheet excellent in phosphate-treatability and corrosion resistance. In addition, the prior art 2 permits the reduction of frictional coefficient of the surface of the continuous-annealed cold-rolled steel sheet.
However, our detailed studies revealed that the prior art 2 had the following problems.
In the prior art 2, when the plating weight of the nickel plating layer is under 5 mg/m.sup.2, a cold-rolled steel sheet excellent in phosphating-treatability is unavailable. The reason is as follows: The number of initially precipitated nuclei of phosphate, which is required for forming a dense and uniform phosphate film and giving a crystal grain size within a certain range by means of the phosphating treatment, is within a range of from 1.times.10.sup.10 to 5.times.10.sup.11 /m.sup.2 in terms of the distribution density.
In order to limit the distribution density of nickel particles in the nickel plating layer within the range of from 1.times.10.sup.12 to 5.times.10.sup.14 /m.sup.2 as described above, however, the plating weight of the nickel plating layer must be at least 5 mg/m.sup.2. According to the prior art 2, however, the plating weight of the nickel plating layer is disclosed to be within a range of from 1 to 50 mg/m.sup.2. Accordingly, when the plating weight of the nickel plating layer is under 5 mg/m.sup.2, it is impossible to achieve a distribution density of the nickel particles of at least 1.times.10.sup.12 /m.sup.2. Therefore the number of initially precipitated nuclei of phosphate cannot in some cases be kept within a desired range described above by the prior art 2, in which case an excellent phosphating-treatability of the steel sheet is unavailable.
In the prior art 2, furthermore, improvement of phosphating-treatability and reduction of frictional coefficient of the surface of the cold-rolled steel sheet are attempted by forming a non-metallic nickel film on the surface of the nickel plating layer. However, non-metallic nickel is basically a metal oxide, and as disclosed in the examples of the prior art 2, when forming a non-metallic nickel oxide film having an average thickness of at least 0.005 .mu.m on the steel sheet surface by subjecting the steel sheet to an anodic electrolytic treatment in an alkaline bath, non-metallic nickel oxide film having an average thickness larger than the above is formed on a portion of the steel sheet surface not having a nickel plating layer. Consequently, although press-formability is improved, the phosphate film contains more portions with a small deposited weight, thus resulting in a lower paint adhesivity and a poorer corrosion resistance after painting.
Because of the low hardness of nickel, improvement of press-formability through the reduction of frictional coefficient of the surface of the steel sheet requires formation of a thicker nickel oxide film on the surface of the nickel electroplating layer. An increased deposited amount of the nickel oxide film leads however to a lower phosphating-treatability.
In the prior art 2, therefore, it is difficult to improve simultaneously press-formability and phosphating-treatability.
When manufacturing a cold-rolled steel sheet for deep drawing by using a mild steel sheet as the material and subjecting same to a continuous annealing treatment, it is necessary to solve simultaneously the two problems of a decrease in phosphating-treatability as well as in press-formability.
Under such circumstances, there is a strong demand for the development of a nickel alloy electroplated cold-rolled steel sheet for deep drawing excellent in press-formability and phosphate-treatability, suitable for the application of the continuous annealing treatment, but such a cold-rolled steel sheet and a method for manufacturing same have not as yet been proposed.